The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2002-3034 filed on Jan. 10, 2002, which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a metal substrate for carrying a catalyst installed in an exhaust system of a vehicle for purifying exhaust gas and a method for manufacturing the same.
2. Description of the Related Art
A substrate for carrying a catalyst is known, which is constructed such that a plurality of honeycomb matrices formed by rolling metallic foil are arranged in series inside a single casing.
According to this technique, in comparison with a substrate for carrying a catalyst whose whole body is formed by a single honeycomb matrix, the divided individual honeycomb matrices is compact, and the heat capacity decreases. Hence, there is an advantage in that the temperature increases at an early period with starting from a honeycomb matrix disposed on an exhaust gas inlet side at a time of the cold starting of the engine, and activation of the catalyst starts.
Such a metal substrate for carrying a catalyst in which the plurality of honeycomb matrices are arranged in series is fabricated by a method in which the individually fabricated honeycomb matrices are brazed to inner surfaces of short outer tubes to form unit substrates, and the outer tubes of the unit substrates are butt welded into a monolith, or a method in which after individual honeycomb matrices respectively having a brazing filler material wound therearound are inserted into one outer tube at predetermined intervals, the honeycomb matrices are brazed to the inner surface of the outer tube upon heating.
However, with regard to the metal substrate based on the former fabrication method, welding must be performed with respect to each unit substrate, so that the fabrication involves time. Also with regard to the metal substrate based on the latter fabrication method, it is difficult to insert the plurality of honeycomb matrices with the brazing filler material wound therearound into the outer tube by maintaining the predetermined intervals.
Furthermore, thermal stress occurs in the honeycomb matrix due to a temperature difference between upstream and downstream of a flow of exhaust gas. Therefore, to avoid a decline in durability due to this difference, when the honeycomb matrix is brazed to the outer tube, it is necessary to limit a brazing portion to a downstream side and release an exhaust gas inlet side from constraint to allow thermal expansion. However, there are problems in that since the length of the honeycomb matrix is short, the brazing filler material is liable to flow, and it is difficult to control the brazing range in any honeycomb matrix, and that since distance from the brazing portion to a free end is short in all honeycomb matrices, the alleviation of stress cannot be substantially expected in all honeycomb matrices.
Accordingly, in view of the above-described problems, the object of the invention is to provide a metal substrate for carrying a catalyst, which can be fabricated easily and in which the alleviation of thermal stress can be realized at low cost, as well as a method of manufacturing the same.
To this end, according to a first aspect of the invention, there is provided a metal substrate for carrying a catalyst including an outer tube, a plurality of honeycomb matrices arranged in series in the outer tube, and an intermediate tube, which is formed by winding a wide sheet made of a metal foil around an outer peripheral surface of the plurality of honeycomb matrices. The intermediate tube is bonded to the plurality of honeycomb matrices. The intermediate tube is brazed in a predetermined region on an outer peripheral surface at an end portion thereof and are bonded to the outer tube.
Despite the fact that a plurality of honeycomb matrices are provided, brazing is affected at one portion, so that the cost can be lowered, and the thermal stress can be reduced reliably.
According to a third aspect of the invention, there is provided a method for manufacturing a metal substrate for carrying a catalyst, the metal substrate having a plurality of honeycomb matrices arranged in series in an outer tube, the method including the steps of winding a wide sheet formed of a metal foil around outer peripheral surfaces of the plurality of honeycomb matrices to form a subassembly in which the plurality of honeycomb matrices are arranged in an intermediate tube formed of the wide sheet, winding a brazing filler material around an outer peripheral surface of the intermediate tube at an end portion of the intermediate tube, inserting the subassembly into an outer tube, and heat processing the outer tube into which the subassembly is inserted.
According to a fourth aspect of the invention, in the third aspect, the heat processing step bonds the plurality of honeycomb matrices to the intermediate tube and brazes the intermediate tube to the outer tube.
Since the plurality of honeycomb matrices are inserted into the outer tube as one subassembly and brazing is affected, the operation is extremely simple.
According to a fifth aspect of the invention, the method of any one of the third and fourth aspects further includes the steps of lapping and winding each corrugated sheet and each flat sheet around a common core bar to form the plurality of honeycomb matrices concurrently, winding the wide sheet around the plurality of honeycomb matrices to form the intermediate tube, and pulling out the common core bar from the honeycomb matrices to form the subassembly.
Sine the plurality of honeycomb matrices are simultaneously fabricated on a common core bar, the honeycomb matrices can be efficiently formed into the same size, and the formation of the intermediate tube can be subsequently affected simply.
According to a sixth aspect of the invention, the method of any one of the third to fifth aspects, further includes the steps of reducing an outer diameter of the outer tube into which the subassembly is inserted, before the heat processing step.
By virtue of the diameter reduction, the degree of contact between adjacent ones of the honeycomb matrices, the intermediate tube, and the outer tube can be promoted, thereby making it possible to ensure more satisfactory bonding or brazing.
According to second and seventh aspects of the invention, the wide sheet is formed into a lap-wound layered form in which a flat sheet and a corrugated sheet are lapped and wound. Consequently, an increase in the strength of the wide sheet can be attained.